Implement basic support of SystemSaveData and Cleanup IFileSystemProxy (#767)

* Implement basic support of SystemSaveData and Cleanup IFileSystemProxy

- Implement `OpenSystemSaveData` as a `IFileSystem` in `SaveHelper`:
  On real device, system saves data are stored encrypted, and we can't create an empty system save data for now. That's why if a user put his own dump of system save in `RyuFs\nand\system\save\`, we extract content in associated folder and open it as a `IFileSystem`. If the system save data don't exist, a folder is created.

- Cleanup `IFileSystemProxy` by adding a Helper class.

- Implement `GetSavePath` in `VirtualFileSystem` and remove `GetGameSavePath` in `SaveHelper`.

* remove the forgotten I

* Fix align

Ryujinx.HLE/FileSystem/SaveDataType.cs

@@ -9,4 +9,4 @@
         TemporaryStorage,
         CacheStorage
     }
-}
+}